Purpose:
To determine the nature of the cellular rearrangements occurring before and after the remodeling zone (RZ) in human natural lens, identified by confocal microscopy to be about 100 µm from the capsule (Lim et al. IOVS 2009).

Methods:
Transparent donor lenses from NC Eye Bank and Ramayamma International Eye Bank, Hyderabad, were fixed initially with 10% formalin followed by 4% paraformaldehyde prior to processing for electron microscopy using the Vibratome sectioning technique. Twenty-seven lenses were fixed (ages 22-92) and lenses of ages 22, 55 and 92 were examined in detail.

Results:
Overview electron micrographs confirmed the loss of cellular organization present in the outer cortex (80 µm thick) as the cells transitioned into the RZ. The transition occurred within a few cell layers and fiber cells in the RZ completely lost their classical hexagonal cross-section appearance. No regular shape was recognizable. Cell interfaces became unusually interdigitated and irregular even though the radial cell columns were retained. Cell membranes were consistently 7 nm thick with constant extracellular space except at gap junctions, which appeared to be unaffected. After the 40 µm thick RZ, the cells were still irregular but more recognizable as fiber cells with typical interdigitations and the appearance of undulating membranes. A major shift in staining density occurred across the RZ with cytoplasm staining light and membranes dark before the RZ, the opposite after the RZ. Cell thickness was irregular after the RZ with some cells compacted while others were not. This band of early irregular compaction in the cortex was over 500 µm thick before the zone of full compaction in the adult nucleus is reached. Within this layer, final organelle degradation occurs, water is lost creating the refractive index gradient and undulating membranes become prominent.

Conclusions:
Massive cytoskeletal rearrangements most likely occur in the RZ releasing the crystallins from bonds to cytoskeletal (intermediate) filaments so that water and protein can reorganize into the uniform dense high refractive index matrix characteristic of the nuclear fiber cells. Cytoskeletal attachments to membrane proteins are also altered to allow orthogonal arrays of aquaporin0 to form as undulating membranes develop. The extent of cellular reorganization in the RZ was surprising.